Electroactive polymers are conducting polymers in which at least one property of the polymer changes when an electric field is applied. These polymers, which are otherwise known as inherently conducting polymers or ICPs, are widely used in industry. For example, highly transparent electroactive polymers can be used in display devices, such as liquid crystal display devices. Highly transparent electroactive polymer thin films are used as electrochromic materials. Electrochromic materials are materials that reversibly change colour when placed in the presence of an electric field.
Various electrochromic materials are known. “Inorganic” electrochromic materials can be formed from certain “inorganic” transition metal oxides, such as tungsten oxide, molybdenum oxide, and vanadium oxide. In the absence of an applied electric field inorganic electrochromic materials are in a neutral state having intermediate transparency and consequently substrates containing the materials display an intermediate transparent or reflective surface. On the other hand, when an electric field is applied to the electrochromic materials they change to a coloured state, thereby reducing the amount of light transmitted through the electrochromic substrate in the case of a transparent substrate such as a window, or the amount of light reflected from the surface of a substrate having a reflective surface, such as a mirror. Removal of the electric field returns the electrochromic materials to their intermediate neutral state.
“Organic” electrochromic materials can be formed from certain polymers. For example, electrochromic polymer films may be prepared by dissolving organic polymers in a solvent, casting or coating the resulting solution onto an electrode substrate, and removing the solvent to form an electrochromic substrate having the electrochromic conducting polymer on the surface of the substrate. An example of an “organic” electrochromic material is polyaniline which can be formed either by the electrochemical or chemical oxidation of aniline. The materials are normally in a neutral state when no electrical field is present, but change to a coloured state when an electric field is applied and conversely change to an uncoloured (sometimes called a “bleached” state) when the electric field is reversed.
Electrochromic materials may be coated onto transparent substrates, such as glass, or onto reflective substrates, such as mirrors, to form electrochromic substrates. Electrochromic devices containing electrochromic substrates as described above are known in the art. Typically, these devices are electrolytic cells including an anodic electrochromic substrate, a cathodic electrochromic substrate, and an electrolyte. The two electrochromic substrates are typically separate and distinct from one another and assembled in a spaced apart relationship.
One application of electrochromic devices has been in the field of rear view mirrors for motor vehicles. Electrochromic rear view mirrors change from a full reflectance mode (day) to a partial reflectance mode (night) for protection from light emanating from the headlights of vehicles approaching from the rear.
Among the different electroactive polymers used in practical applications, poly(3,4-ethylenedioxythiophene) (PEDOT) is a stable electroactive conjugated polymer. PEDOT can be formed by oxidatively polymerizing 3,4-ethylenedioxythiophene (EDOT) monomer by wet chemical polymerization, electrochemical polymerization or vapour phase polymerization. A common method for obtaining PEDOT films is to polymerize EDOT via wet chemical oxidation and then apply the polymer to a substrate using a suitable coating process, such as dip coating, spin coating, printing, spray coating, etc. A difficulty with this approach to film formation is that PEDOT is difficult to maintain in solution. In response to this difficulty polyelectrolytes, such as poly(styrenesulfonate), have been used to form stable PEDOT:polyelectrolyte suspensions. However, the use of a polyelectrolyte can adversely affect the conductivity of the PEDOT film.
Electrochemical polymerization can also be used to deposit PEDOT films on substrates. Thus, EDOT monomer can be coated onto a conductive substrate and the sample placed into a three electrode cell with an electrolyte. A periodic voltage can be cycled across the cell, with each cycle increasing the amount of PEDOT deposited on the substrate. After 20-40 cycles film growth is generally finished. Whilst the PEDOT films formed using this method are generally high in quality, the deposition technique only allows for PEDOT films to be formed on conductive substrates and the technique is not well suited to large scale and/or commercial applications.
PEDOT films have also been formed on substrates by vapour phase polymerization of EDOT. In this method, a substrate is coated with an oxidant and the oxidant coated substrate is placed in to a chamber containing EDOT monomer in the vapour phase. Under appropriate conditions the monomer condenses on the substrate and polymerizes, creating a highly conductive and homogenous PEDOT film.
Vapour phase polymerization methods were initially developed using FeCl3 or H2O2 as oxidant in the formation of polypyrrole films. Subsequently, iron(III) tosylate was used as oxidant in the vapour phase polymerization of pyrrole. More recently, iron(III) tosylate has been used as oxidant in the vapour phase polymerization of EDOT to produce PEDOT films. However, the homogeneity of films made via vapour phase polymerization has tended to be inferior to those made by solvent casting. It has been suggested (Winther-Jensen, et al., Macromolecules 2004, 37, 5930-5935) that the poor film quality obtained using FeCl3 and/or iron(III) tosylate in the vapour phase polymerization may result from the formation of crystal-like structures in the oxidant layer. It has also been found that the oxidant iron(III) tosylate produced an undesirable polymerization route for EDOT which resulted in partially conjugated polymer chains.
There is a need for processes for producing PEDOT films, and other polyaryl or polyheteroaryl polymer films, that overcome one or more of the aforementioned problems with known processes. Alternatively, or in addition, there is a need for processes for producing PEDOT films, and other polyaryl or polyheteroaryl polymer films, that have a commercially acceptable conductivity for use in electrochromic devices. Alternatively, or in addition, there is a need for processes for producing PEDOT films, and other polyaryl or polyheteroaryl polymer films, having commercially acceptable conductivity to produce relatively large scale electrochromic devices, such as windows. Alternatively, or in addition, there is a need for processes for producing PEDOT films, and other polyaryl or polyheteroaryl polymer films, on substrates other than glass, such as plastic substrates.
Before turning to a summary of the present invention, it must be appreciated that the above description of the prior art has been provided merely as background to explain the context of the invention. It is not to be taken as an admission that any of the material referred to was published or known, or was a part of the common general knowledge.